1. Field of the Invention
The present invention relates to dielectric compositions useful as insulating material for electrical equipment, particularly as liquid dielectric impregnants for capacitors.
2. Description of the Prior Art
Liquid insulating or dielectric compositions are used extensively in the manufacture of various electrical devices. For example, electrical devices such as capacitors, transformers, circuit breakers, cables and the like require a liquid dielectric composition having high permittivity, good thermal stability and non-inflammability. Halogenated aromatic compounds, specifically chlorinated diphenyl and chlorinated naphthalene, since they possess many of the desired characteristics, have been widely used as electrical insulating material in electrical equipment. A difficulty arises in the use of these halogenated aromatic compounds when after extended periods of time electrical discharge, elevated temperatures, high voltage or chemical hydrolytic attack may cause partial decomposition of these compounds. The decomposition products are generally hydrogen halides including hydrogen chloride which adversely affects the properties of the dielectric material and corrodes the various components of the apparatus.
The undesirable effect of the above decomposition is further manifested when the above halogenated aromatic compounds are used as capacitor dielectric impregnants. Thus, under high operating temperatures and direct current voltages, capacitors impregnated therewith undergo a characteristic type of deterioration which results in an ever increasing leakage current, a short capacitor life, visible localized decomposition of the dielectric and corrosion of the electrodes. Moreover, in the case of capacitors operating on alternating current, the decomposition sometimes results in an excessive increase in power factor of the dielectric material.
Furthermore, the liquid dielectric compositions employed in insulating the electrical apparatus must combine a certain number of properties such as low initial conductivity, conductivity after aging, resistance to thermal decomposition, resistance to decomposition by hydrolysis, relatively low viscosity, low vapor pressure, and low freezing point with the ability to vary the permittivity characteristic to meet specific electrical component design requirements.
Accordingly, a need exists to provide a liquid dielectric composition retaining the desired properties mentioned above whereby the deleterious action of hydrogen halides on the various components of the electrical apparatus is eliminated or materially reduced. A need also exists to provide the desired properties over extended periods of time and under widely varying conditions.